Brazil President Dilma Rousseff is facing one of the defining moments of her presidency as pressure builds on her to veto a bill that would open vast protected areas of forests to ranching and farming, potentially reversing Brazil’s major gains in slowing Amazon deforestation.

Rousseff, who has until May 25 to make her decision, has not commented publicly on whether she will veto the bill. But senior officials, including Mendes Ribeiro Filho, the agriculture minister, have signaled that parts of the bill were unacceptable, suggesting that one option might be a line-item veto of parts of the bill.

The Forest Code, which Congress approved in April at the urging of powerful agricultural groups, is an effort to overhaul Brazil’s 47-year-old legislation providing forest protection. The bill has emerged as a highly sensitive issue for Rousseff ahead of the U.N. Conference on Sustainable Development, scheduled to be held here next month.

The bill would effectively give amnesty to landowners who illegally deforested areas before 2008, drawing the ire of environmentalists. If the legislation goes into effect, it could allow landowners in the Amazon to reduce obligatory forest cover to 50 percent from 80 percent, and could lead to the loss of as much as 190 million acres of forest, according to the government’s Institute for Applied Economic Research.

The bill’s arrival on the president’s agenda comes at a delicate time for Brazil’s government, forcing it to examine its alliances with parties that supported the bill. Brazil is already facing scrutiny over plans for huge energy projects in the Amazon, which Rousseff has defended. Worker revolts have recently flared at some dam construction sites, while strikes have slowed work at Brazil’s largest hydroelectric project, Belo Monte.

“Brazil cannot be allowed to take this step backward,” Marina Silva, a former environment minister and presidential candidate, said in an interview. “Obviously, giving amnesty to those who have destroyed forests heightens the risk of new deforestation.”

Equally adamant in their support of the bill are Brazil’s “ruralistas,” legislators representing agricultural interests. They are contending that the new Forest Code is needed to support Brazil’s economy, which draws strength from exports of agricultural products like beef, soybeans, sugar and poultry.

Katia Abreu, a senator representing Tocantins state and the president of the National Confederation of Agriculture and Livestock, lashed out at international environmental groups that have been petitioning Rousseff to veto the bill.

“There are NGO’s out there that are compromised with their countries of origin, particularly from Europe,” said Abreu, referring to nongovernmental organizations and describing their actions as an “attempt to paralyze the growth of Brazilian agribusiness.”

Still, other prominent voices in Brazil, which is home to about 40 percent of the world’s rain forests, have weighed in against the new Forest Code, including the Brazilian Academy of Sciences and the Brazilian Society for the Advancement of Science, two of the country’s leading scientific groups. Anger over the bill has spread into popular culture. Exemplifying the sentiment in the entertainment industry, the actress Camila Pitanga broke protocol at an event this month, calling on Rousseff, who was present, to veto the bill. Video images of Pitanga’s statement spread quickly on social media throughout Brazil.

Stunning some of the ruralistas, support for a veto has also emerged among some corporate leaders in Sao Paulo, Brazil’s business capital. Valor Economico, the country’s top financial newspaper, compared the moment to the battle over US President Barack Obama’s health care law, calling Rousseff’s choice “one of those decisions which defines a government.”

“This bill leaves Brazil in the Middle Ages,” said Paulo Nigro, president of Tetra Pak Brasil, a food packaging and processing company, who was one of several prominent Sao Paulo business leaders quoted by Valor voicing their opposition to the Forest Code.

Still, the dismay in some quarters over the approval of the bill weeks before thousands of environmentalists flock here for the United Nations’ sustainable development conference, called Rio Plus 20, belies an important shift in Brazilian politics, illustrated by the rising power of the ruralistas.

This voting bloc of more than 100 lawmakers chafed at Rousseff’s assertion during her campaign for president in 2010 that she would not support legislation that offered amnesty for illegal forestation. The ruralistas also lured lawmakers from the Brazilian Democratic Movement Party, a pillar of Rousseff’s coalition, to support the Forest Code.

“It’s embarrassing enough for the host country of Rio Plus 20 to be facing this kind of situation,” said David Fleischer, professor emeritus of political science at the University of Brasilia. “But Dilma also underestimated the ruralistas, who are showing their clout,” Fleischer said, referring to Rousseff by her first name, as she is widely known in Brazil.